Machine tools are tested to determine their true operating values and their geometrical precision by means of standardized and comparable measuring methods before being placed in operation and thereafter periodically. The accepted and controlled methods used, e.g., for the geometrical precision, are set forth as measurement and test specifications and standards. These standards are mandatory and provide a uniform measurement and test procedure such that the results of measurements from different sources can be compared.
Typical standards are the German provisional standard DIN 8601 "Acceptance Conditions For Machine Tools For Cutting Metals", the guidelines of the Verein Deutscher Ingenieure VDI, and the Deutsche Gesellschaft fur Qualitat DGQ which, in guideline No. 3441, defines the bases for the "Statistical Testing Of The Working And Positioning Accuracy Of Machine Tools". Other standards in this field are the United States National Aerospace Standard NAS 985 "Uniform Alignment Tests" and NAS 979 "Uniform Cutting Tests". A third standard known on a worldwide basis is the draft international standard DIS of the International Organisation for Standardization ISO, published as ISO/DIS 3070/0 "Test Conditions For Boring and Milling Machines With Horizontal Spindle--Testing The Accuracy (General Introduction)".
These conventional methods use a number of different specifications and standards for determining the actual states of machine tools, including the following for machine acceptance and for checking the geometrical precision or accuracy of the machine tool:
(1) Measurement of the individual machine components, e.g. by laser interferometers, electronic spirit levels, autocollimators, etc. However, such devices are complicted and expensive to obtain, operate and maintain. They also fail to provide details of the spatial error field, and thus, of all the component errors of the machine tool, because all the component errors cannot be measured simultaneously. PA1 (2) Manufacture of test workpieces on the machine tool to be tested and subsequent measurement of this test piece on a measuring machine. This indirect method often fails to provide a clear and unambiguous indication of the relationship between the error on the test workpiece and the cause of the error. The conclusion drawn is often ambiguous. Also, the superimposing of the effects of the machine tool caused by variations in the workpiece, tool materials and tool state complicate the testing method. PA1 (3) Measurement of test workpieces on machine tools, which are fixed in the machine tool in place of a workpiece. Probes or sensors working in various dimensions are used in place of the tool, and their deflections during operations are measured. A calculation employing the machine tool coordinates permits a quantitative determination of variations in the geometrical accuracy of the machine tool. This procedure obviates the problem of material variations and variations in the state of the tool. However, problems arise in coorelating the deflections on the sensor or probe with the coordinates of the machine tool. Generally, the machine tool coordinates must be manually incorporated.
If a machine tool has one or more geometrical errors or faults, it will not produce an exact circular curve. The divergence of the machine tool from the ideal geometry is indicated by variations from the circular shape. The circular shape test provides satisfactory information of the machine tool precision in one plane. If a sensor is used in the measurement on a machine tool, the influence or effect of the manufacturing process, ie.e, the interaction between material and tool is eliminated. However, the problem is incorporating the machine tool coordinates into the evaluation is still present. Here again, such coordinates generally must be manually associated in a very timeconsuming manner with the sensor coordinates.